A semiconductor element (chip) is usually, for protection and blocking from an external environment (such as external air, contaminant, light, magnetism, high frequency wave or impulse), sealed with a resin (mold resin) and is mounted, on a substrate, in the form of a semiconductor package having the chip accommodated therein. Typically, the package is one formed by a transfer molding wherein a thermosetting resin (mold resin) such as an epoxy resin is melted by heating, and then, it is transferred in a mold where the semiconductor chip is set, followed by filling and curing. To the mold resin, a releasing agent is added in addition to a curing agent, a curing accelerator and a filler, to secure smooth releasability of the formed package from the mold.
On the other hand, along with a strong demand for improvement of productivity of a semiconductor package, there is a problem such that since the resin adheres to the mold, the contaminated mold needs to be often cleaned, or when a sealing resin having a low shrinkage corresponding to a large package is used, sufficient releasability cannot be obtained even if a releasing agent is added. Therefore, there has been a development made for a technique using a release film for resin molds (hereinafter referred to simply as a “release film”), wherein a semiconductor package is formed by injecting the resin into the mold in such a state that the resin molding portion (cavity surface) of the mold is covered with the release film without directly contacting the sealing resin to the cavity surface of the mold, and such technique has shown results of a certain level (e.g. Patent Documents 1 to 3).
However, recently, the mold resin to be used for a package of a semiconductor element is required to be changed to a non halogenated mold resin for environmental protection. Further, in order to meet the requirements for forming a semiconductor into a fine pitch model, slim model, multilayer chip package model or LED, there has been a further progress in forming a low viscosity mold resin or forming a mold resin into a liquid form resin. As a result, in the step of molding a resin for the semiconductor element, the amount of gas and low viscosity material formed from the molten mold resin under a high temperature environment increases, and the gas and low viscosity material permeating through the above release film for molds contact the mold having a high temperature, whereby the mold contamination becomes intense.
Further, covering the mold surface with the release film is carried out by sucking and supporting the film thereon under vacuum, but a volatile component such as an oligomer in the film transfers to the covered side of the mold, whereby the mold contamination may sometimes be caused.
Thus, even though the release film is used in such a manner, the side of the mold covered with the film, tends to be easily contaminated. Further, once the contamination is caused, the molding step for the semiconductor is required to be stopped for cleaning, and such leads to a problem such that the production efficiency of the semiconductor decreases.
Further, from such a viewpoint, the above Patent Documents 1 and 2 describe that in order to reduce the permeation of contaminants, a vapor-deposited layer of a metal or a metal oxide is formed on one side (a surface to be contacted with a mold surface) of the release film. However, such a metal deposited layer is one to be used by direct physical contact to the mold surface, and a metal powder tends to be released from the film surface or a broken section of the film, whereby its use for a molding step of the semiconductor resin is limited.
Further, in Patent Documents 1 and 2, the gas permeability of the release film is defined by a permeation of carbon dioxide gas, but such is not valid as an index for evaluating the permeability of a low viscosity material, etc. from a resin, etc.
Further, the release film is now required to have higher releasability from the mold resin, but such was not considered for the above release film, and the releasability was insufficient.
Further, in a case where a mold having a large surface roughness was used, when the release film is vacuum sucked to the mold before resin sealing, the release film was sometimes required to have a mold obsequence property whereby the release film is capable of being sufficiently stretched along the circumference length corresponding to the surface roughness of the mold.
Patent Document 1: JP-A-2002-361643 (Claims 1 to 3)
Patent Document 2: JP-A-2004-79566 (Claims 1 to 3)
Patent Document 3: JP-A-2001-250838 (Claims 1 to 6)